How to trace the path of an object in the video editor?
Combining colored elements is easiest in the compositor.

I would also suggest using 3D to create the colored line as Blender has good visual tools in 3D. Just use a Shadeless or emitting material to make it monochromatic.

Then mask out the part of the bird, which shouldn't be covered by the line.



Render it with transparency and use a mix node in the compositor. Check the invert button in the mask options (in the image editor) to invert the mask.